Core/shell semiconductor quantum dots (Qdots) have attracted enormous research interest in the field of electronics, optoelectronics and bioimaging. To further diversify core/shell Qdot applications, especially in the field of spintronics, dopant based Qdots are of particular interest. By doping traditional CdS or ZnS core with transition metal ions such as manganese (Mn), cobalt (Co), nickel (Ni) and the like, it is possible to develop dilute magnetic semiconductor (DMS) Qdots for spintronic applications as reported in recent scientific articles written by D. A. Allwood et al in Science 2005 Vol. 309, pages 1688-1692 and P. I. Archer in Nano Letters 2007 Vol 7, pages 1037-1043, for example.
It is expected that these dopant based binary DMS Qdots will advance the capability of the next generation memory storage devices and computers. In this context, it is important to synthesize dopant based ternary DMS Qdots, such as manganese (Mn) dopant based CdxZn1-xS:Mn/ZnS Qdot heterostructures.
Fluorescent quantum dots (Qdots) have demonstrated their potential in diagnostic bioimaging applications in vitro as discussed by H. Yang et al. in Advanced Materials 2006, Vol. 18, page 2890. For in vivo bioimaging applications, however, the embodiment of additional properties such as paramagnetism into the same fluorescent probe is highly desirable. These multimodal probes would benefit in vivo disease diagnosis and surgical guidance based on their ability to be detected in multiple modes, such as, optically and magnetically. Thus, synthesis of bright multimodal Qdots is a matter of great interest to a broad area of the scientific community from physics to bioscience.
The wide band gaps of the II-VI group semiconductors such as CdS and ZnS serve as good host materials for various kinds of foreign elements known as dopants. Out of the different transition metals, manganese usually occupies cadmium (Cd) or zinc (Zn) substitutional sites in the host lattice as a divalent ion. The excitation and decay of manganese ion produces a yellow/orange luminescence at approximately 590 nm wavelength, as reported by R. N. Bhargava et al in Phys. Rev. Lett. 1994, Vol. 72, page 416 and S. Biswas et al. in Journal of Physical Chemistry B, 2005, Vol. 109, 17256. This emission peak is generally associated with a transition between 4T1 and 6A1 energy levels. Also, the presence of the Mn2+ ions within the host Qdots introduces the paramagnetic property.
The realization that many molecular phenomena result in mechanical responses at the nanoscale level promises to bring about a revolution in the field of chemical, physical, and biological applications. In a quest for smaller, faster, better, more accurate measuring and analytical devices there has been a wide application of traditional dopant based Qdots, particularly in biomedical imaging. However, the applications are limited because of a relatively narrow excitation range, typically in the UV range between 200-375 nm wavelengths.
In spite of the many advantages of dopant based quantum dot semiconductors disclosed in scientific applications today, there are limitations and disadvantages of the existing quantum dots regarding their adaptability and reconfigurablility. For example, traditional dopant based Qdots in biomedical imaging with a relatively narrow excitation range between 200-375 nm wavelengths is extremely harmful for biological systems as excitation in this wavelength range can easily destroy live cells. Again, due to narrow excitation ranges, these Qdots will not be efficient for capturing a broad spectrum of solar light. Ideally, Qdots with broad excitation bands will eliminate the above-mentioned limitations of traditional dopant based Qdots.
The performances of the QDots are often influenced by their surfaces since an appreciable portion of the constituent atoms reside at their surfaces for example, for a QDot with a diameter of 4 nm, 30% of its atoms reside on the surface and thus are missing one or more of their four (tetrahedral) bonds to neighboring atoms. Chemically passivating these surface atoms and providing them with a true tetrahedral bonding environment plays a significant role in determining the optical and electronic properties of the QDot.
A significant improvement in the performances of the QDots was realized by growing a semiconductor shell around the core compared to the organic surface capping ligands traditionally used to chemically passivate the QDot surface as discussed by H. Yang et al. in Advanced Functional Materials 2004, 14, 152 and H. Yang et al. in Journal of Chemical Physics 2004, 121, 10233. Of the various types of QDots, Mn-doped II-VI QDots are of special attraction owing to their bright luminescence at room temperature in the visible region. Also, presence of the Mn ion as a transition metal ion in the semiconductor host make them dilute magnetic semiconductor (DMS), an interesting materials for application in the field of spintronics. Especially, the Mn doped type I core-shell semiconductors are suitable for bio-imaging applications due to the large Stoke's shift in the emission spectra as discussed by S. Santra et al. in Chemical Communications 2005, 3144; S. Santra et al. in Journal of the American Chemical Society 2005, 127, 1656, and H. S. Yang et al. in Advanced Materials 2006, 18, 2890.
Performances of these doped semiconductors both as fluorescent as well as spintronics materials depends on the position and distribution of the Mn atoms inside the host lattice. Doping in the semiconductor nanocrystals are often encountered with various difficulties due to various reasons including various kinetic factors, preferential adsorption through specific surfaces etc. Difference in the ionic radii of the substituent dopant and the substituted cation often introduces significant amount of strain in the nanocrystal lattice; since strain fields are necessarily long range, much longer than typical nanocrystal dimensions, it tends to relieve itself by ejecting the dopant to the surface of the nanocrystals. Thus, it is extremely important to investigate the positions of the Mn atoms inside a traditional CdS:Mn/ZnS core/shell QDot.
The dopant based core-shell semiconductor quantum dots (Qdots) of the present invention solve many problems and overcome many limitations in the prior art through alloying between the CdS and ZnS phase and providing a different atmosphere to the substituent dopant.